This is a user-to-user technical community. Contributors here are all end-users, just like you. Other than the site Moderators, Apple neither monitor nor participate within the Support Communities.
Apple invite submission of feature requests via its Product Feedback portal:
Feedback - iPad - Apple
Some mailbox folders don’t have a Select All button - however, there is an alternative…
From the mailbox sidebar, tap the Edit button at top-right of the Mail sidebar; simply swipe your finger down (or up) the left-side of the message list to bulk select Mail Messages. Swipe below the bottom edge to rapidly select all the visible email messages.
Once selected, tap the Delete button at the bottom of the sidebar.